The specific aims of this proposal are to characterize the properties of the lymphocytes and macrophages recovered from lungs that have been infected with Mycobacterium bovis BCG. The lymphocytes will be characterized with respect to whether they are large, short-lived cells or small, predominantly long-lived cells. The proportion of T-lymphocytes will be determined, and these cells will be divided into subsets with respect to the surface markers Lyt 1, Lyt 2, 3 and Ia. The functions of these cell populations will be assayed with respect to adoptive transfer of tuberculin delayed type hypersensitivity and antituberculosis immunity, and antigen-induced transformation in vitro. Lung macrophages will be studied with respect to their microbicidal activity and release of hydrogen peroxide in vitro. These studies comprise the current phase of a project whose objectives are to discover why the lungs are susceptible to tuberculosis infection and the persistence of such infection despite the expression of adequate levels of systemic cell-mediated immunity. Paradoxically, such infected lungs are highly resistant to infection and the mechanism of this heightened resistance is being elucidated. These studies bear directly on the problem of human pulmonary tuberculosis, which is the most important communicable disease, world-wide.